Viscous isotropic phases, surfactants

At relatively low concentrations of surfactant, the micelles are essentially the spherical structures we discussed above in this chapter. As the amount of surfactant and the extent of solubilization increase, these spheres become distorted into prolate or oblate ellipsoids and, eventually, into cylindrical rods or lamellar disks. Figure 8.8 schematically shows (a) spherical, (b) cylindrical, and (c) lamellar micelle structures. The structures shown in the three parts of the figure are called (a) the viscous isotropic phase, (b) the middle phase, and (c) the neat phase. Again, we emphasize that the orientation of the amphipathic molecules in these structures depends on the nature of the continuous and the solubilized components. 

A third lyotropic mesophase which occurs frequently in surfactant-water systems is normally designated viscous isotropic. This phase is very viscous, sometimes brittle, but unlike neat and middle it is not bire-fringent. The structure of the viscous isotropic phase is still not known with certainty. In some systems x-ray studies have indicated that the structure consists of spherical units packed in a face-centered arrangement 4, 22). It has been proposed that the polar groups of the molecules cover the outside surfaces of the spherical units and that the hydrocarbon chains are essentially liquid in their arrangement inside the units. In this respect the structure is similar to one of the proposed middle phase structures (4). As in the other lyotropic phases, the solvent probably fills the voids among the spherical units of surfactant. 

Zwitterionic surfactants broadly resemble nonionic materials but because two bulky charged groups are involved the head groups are large. An exception is dimethyldodecyl amine oxide [146] which has a compact head group. This forms Hj, Vj and Emphases. The Hi (termed middle phase in this paper) exists between 35 and 65% surfactant and from <20°C to >105 °C. The Vi (viscous isotropic) phase has a narrow range... 

Between and Tq at relatively high surfactant concentration are the neat and middle phase liquid crystalline regions. In addition. X-ray diffraction studies [16] have revealed several other phases intermediate in composition between the neat and middle phases, including the viscous isotropic phase. A comprehensive account of the structure of these and other mesomorphous phases is given by Skoulios [18]. 

Table VI shows the results of polarized light microscopic observations. Sometimes isotropic regions and the middle phase exist simultaneously. The region of the middle phase is marked by heavy lines. The range of the especially viscous middle phase narrows with transition from two to three oxyethylene groups in the surfactant molecule. Up to 27 %, the system appears optically isotropic. In this concentration range the viscosity can be increased strongly by addition of NaCl, as shown in table VII.

FIG. 8.8 Schematic representations of surfactant structures in (a) viscous isotropic, (b) middle, and (c) neat liquid crystal phases. 

At higher temperatures and water concentrations, the system may shift into the cubic mesophase structure (see Figure 15). The water is present as spheres totally surrounded by monoglyceride. This phase has a high viscosity and is sometimes called viscous isotropic in the hterature the two terms refer to the same structure. In the presence of more water than can be accommodated in the internal spherical phase, one obtains a mixture of lumps of this cubic structure dispersed in excess water. With a saturated monoglyceride such as GMS, the lamellar structure is the main mesophase found under practical conditions, while with unsaturated monoglycerides this cubic phase is the predominant one at lower temperatures. At lower water concentrations, the spherical water micelles are farther apart, so the viscosity of the mixture becomes lower, approaching that of melted pure surfactant. This is the fluid isotropic mesophase, sometimes referred to as the L2 phase. 

The closed loop is not the only characteristic of the nonionic surfactant-water binary phase diagram. Like the ionic surfactant-water mixture, nonionic surfactants, at higher concentration in water, exhibit lyotropic mesophases. Figure 3.14 shows a typical binary phase diagram exhibiting the full lyotropic mesophase sequence II, cubic isotropic phase HI, direct hexagonal phase (middle phase) VI, special cubic ( viscous phase) La, lamellar phase (neat phase). Note the presence of the two-phase domains surrounding each mesophase, the critical point on top of each, and the zero-variant three-phase feature. 

A third category of mesophases formed by surfactants comprises the cubic phases. They are also known as viscous isotropic . As the name implies, these phases are based... 

The samples in Fig. 4 show phases of 5% LA070 with 50 mM octanol in DMSO/water mixtures. Up to a concentration of 25% DMSO, the samples are in a two-phase Li/ La-state, and from 30% to 45% DMSO, the samples are in a single La-phase. With 50% DMSO, the samples are in a two-phase situation. With even higher DMSO concentration, all samples are in a low viscous isotropic state. It is likely that at such high DMSO concentration DMSO has increased the critical micelle concentration (CMC) of the surfactant and co-surfactant concentration so much that micellar aggregates are no longer stable and the phases are molecular solutions. It is obvious that the concentration of 30% DMSO can swell the phases even... 

In some surfactant systems, more complex phase behavior involving one or more viscous isotropic structures will appear. Such phases usually exhibit an X-ray pattern characteristics of a cubic lattice. Such phases are now recognized as the primarily cubic bicontinuous phases introduced above. More recent research with bulky surfactant molecules has led to the suggestion of wormlike or ribbon micelles that may be best described—conceptually, at least—as super aggrega-tions of smaller miceUar units or twisted hexagonal systems. 

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [411]. For example, in some cases the addition of a small amount of non-ionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer, which is possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [25,110], In general, some very stable foams can be formed from systems in which a liquid crystal phase is present at lamella surfaces and in equilibrium with an isotropic interior liquid. If only the liquid crystal phase is present, stable foams are not produced. In this connection foam phase diagrams may be used to delineate compositions that will produce stable foams [25,110],... 

Microemulsions are clear (transparent and translucent are also used in the literature), thermodynamically stable, isotropic liquid mixtures of oil, water, and surfactant, frequently in combination with a cosurfactant. The aqueous phase may contain salt(s) and/or other ingredients, and the oil may actually be a complex mixture of different hydrocarbons and olehns. In contrast to ordinary emulsions, microemulsions form upon simple mixing of the components and do not require high shear conditions generally used in the formation of ordinary emulsions. Microemulsions tend to appear clear due to the small size of the disperse phase. However, clear appearance (transparency) may not be a fundamental property. Sometimes microemulsion may not look clear to the naked eye in the case where dark viscous oil exists. The solution may not be purely transparent because it contains aggregates of micelles. Quite often, we still use these terms, even in this book. Probably we should simply use the term homogeneous solution. 

In a study by Kahn et al. [9] on a polydisperse octadecyl amide with nine oxyethylene units, the pure surfactant is found to be a viscous liquid at room temperature. Upon solubilization in water, a clear isotropic solution phase is observed for all concentrations between the cloud point and the solidification temperature. No liquid crystal formation is observed. 

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